Josephson effect in Nb/Al2O3/Al/MgB2 large-area thin-film heterostructures

Abstract

We report the demonstration of dc and ac Josephson effects in Nb/Al2O3/Al/MgB2 thin-film heterostructure. The heterostructure exhibits moderately hysteretic current–voltage characteristic with a dc Josephson current branch and regular microwave-induced Shapiro steps. From conductance spectrum, a gap of about 2 meV at 7.7 K is estimated for the proximized surface of MgB2 electrode.

Subharmonic gap structures and Josephson effect inMgB2∕Nbmicroconstrictions

“…I C = 290 µA and I C = 110µA were measured in these junctions with R N = 7.4Ω and R N = 18.5Ω, respectively. This implies I C R N = (2.1 ± 0.1) mV at T=4.5K, among the highest values reported in the literature 12,14,27 . Our values are very close to those predicted when the superconducting Nb couples with the MgB 2 π-band 21 and follow the expected Ambegaokar-Baratoff temperature behavior 21,30 .…”

“…In comparison with the theoretical predictions 21 , the majority of the reports indicate depressed values of such product and different temperature behaviors irrespectively to the weak link or tunnel nature of the junctions 12,14,26,28,29 . To the best of our knowledge, in the majority of the cases coupling of the conventional superconductor with only the 3D band of the multi-gap material is inferred.…”

“…In the experiments, however, effective Josephson coupling of the 2D band σ with a 3D band, both of a MgB 2 or of a conventional superconducting counterelectrode, has been not observed. In addition to this, the measured I C R N values are often severely depressed and regardless to the junction orientation, different temperature dependencies of the Josephson current are often reported 12,14,26,27,28,29 . Similar behavior is expected in proximity coupled S-I-N-S structures 30 , and in MgB 2 junctions it has been attributed to degradation of T C at the interface and/or to the barrier nature and quality.…”

“…The unusual consequence of this band structure makes that in the clean limit 6 two different energy gaps are formed at the Fermi level, both closing at the same temperature T C . Indeed, two superconducting energy gaps have been experimentally observed by different techniques, including tunneling spectroscopy 7,8,9,10,11,12,13,14,15 specific heat measurements 16,17 , Raman spectroscopy 18,19 , and high-resolution photoemission 20 . The majority of these studies agree with the presence of a larger gap around ∆ σ =7.0÷7.5 meV attributed to the 2D σ-band and a smaller gap ∆ π =2.0÷2.8 meV due to the 3D π-band.…”

Mg B 2 tunnel junctions with native or thermal oxide barriers

“…3 Several groups have already demonstrated tunnel junctions with both lower T c superconductors [4][5][6][7] and with MgB 2 as the top electrode [8][9][10] using different barrier layers such as AlN, MgO, Al 2 O 3 , and the thermal oxide. The results indicate that key issues for fabricating tunnel junctions are the uniform coverage of the base electrode by these barriers, as well as the stability of these barriers when the top MgB 2 electrode is deposited at temperatures ≥300 o C. In junctions with AlN barriers 9 the data suggested that a thermal oxide must be part of the barrier, and that stimulated our work.…”